(1) Field of the Invention
The present invention relates to communications systems, and in particular to a digital telephone communications system having the capability of exchanging both digitized voice and digital data signals. The invention further relates to Integrated Services Digital Networks (ISDNs). In particular, the invention relates to a telephone communications system having a common equipment unit employing a plurality of voice and data buses allowing both voice and digital data transfer and employing a serial digital packetized transmission protocol to and from telephones connected to the common equipment unit.
A need exists in the area of telephone equipment for a system which allows both voice and data signals to be exchanged. For example, it would be advantageous if, in a telephone system, a data terminal or computer could be connected to the telephone system so that digital data signals could be exchanged over the telephone system along with voice signals without interfering with voice communications. For example, a telephone system which also functions as a digital data network within the area in which the telephone system is located would be particularly useful for the transmission of digital data between digital data devices such as computers, printers and terminals. An important requirement of systems of this type is that the transmission of digital data signals does not interfere with the transmission of voice signals and vice versa, and that the transmission of digital delay signals does not reduce the capacity for voice switching.
(2) Description of the Prior Art
Applicants are aware of the following U.S. patents which relate to various features of telephone systems:
______________________________________ 4,390,982 4,092,501 4,280,217 4,390,986 4,160,131 3,715,505 3,924,077 4,288,870 4,412,102 4,476,559 4,456,989 4,408,323 4,389,544 4,432,089 4,394,757 4,381,427 4,338,495 4,511,767 4,171,467 4,061,887 4,314,109 Re. 29,884 .sup. 4,339,633 4,234,765 4,027,110 3,334,181 4,112,261 4,313,036 4,309,765 ______________________________________
U.S. Pat. No. 4,390,982 discloses a digital private branch exchange system which uses a single processor to control the system. As shown in FIG. 2 of this patent, a group bus is employed which comprises two PCM buses. Each bus has provision for 32 channels. Of the available channels, a number are allocated to supervisory tones and common service features. Others are allocated both for conference purposes or for normal calls and still other channels are usable only for normal calls. All stations, trunks and options have access to both buses. As shown in FIG. 4 of this patent, a plurality of the channels on the first bus are available for use in handling calls while the remaining channels are allocated to specific call processing tones, such as dial tone, receiver off hook, etc. Each channel has eight time slots for an 8 bit word in that channel. On the second PCM bus, the channels are allocatable to conferencing with four three-party conferences and one four-party conference possible. The receive data for a conference uses the first three channels while the transmitter data uses the final three channels of the six channels required for a three-party conference. See the lower portion of FIG. 4 of this patent. The final eight time channels of the second bus are usable for four-party conferencing, four channels for receiving data and four channels for transmitting data. Although the '982 patent discloses the use of more than one bus for handling PCM voice data, it does not provide a system wherein a separate bus is provided for handling computer digital data, nor a system wherein the PCM voice data bus can also be used for handling computer data.
U.S. Pat. No. 4,381,427 descrribes a subscriber loop system for voice and data. In association with a time division multiplex (TDM) telephone system, the subscriber loop system provides voice and data communication with a remote telephone station set and an associated data port. The subscriber loop system includes a voice path and a data path coupling the telephones into the telephone system. The TDM telephone system comprises a plurality of separate buses, one of the buses being a TDM signal path into the system and the other bus being a TDM signal path out of the system.
U.S. Pat. No. 4,339,633 for a modular telecommunications system describes a system in which all systems control data is transmitted on one multiple conductor bus used to provide control data transfer. All speech and voice frequency data is transmitted on a second multiple conductor bus between modules of the system. This is shown in FIG. 5 of the '633 patent.
U.S. Pat. No. 4,112,261 for a key telephone system and method, shows a key telephone system having a key service unit comprising a talk pair bus and a control bus. Voice information is carried on the talk pair bus and control information is carried on the separate control bus.
U.S. Pat. Nos. 4,394,757 and 4,389,544 disclose digital telephone systems which utilize a frame format including a "superframe" formed of a plurality of data frames, each preferably comprising three 8-bit data words. The first data word of the frame includes a synchronization code and a signaling code, and the second and third data words include PCM speech samples or other digital data. According to this patent, the use of a multiword frame format permits the telephone apparatus to support two different telecommunications connections simultaneously. For example, as indicated in column 6 of the patent at lines 29 to 38, one connection may be made with another telephone subscriber for a voice communication while another connection is made with a data system for the transmission of digital data. Alternatively, the telephone apparatus may support a first subscriber to subscriber connection for normal voice communication plus a second subscriber to subscriber connection in a call-back function. The frame format is shown in FIGS. 2 and 3 of the '757 patent. As shown, the data is transmitted in serial form. According to the '757 patent, the telephone apparatus simultaneously transmits and receives one frame of information every 125 microseconds, i.e., the standard PCM sampling rate for telephone systems. Each signal frame is divided into at least two equal 8 bit time slots, one time slot for synchronization and signalling information and at least one, but preferably two, time slots for separate independent channels carrying PCM voice data and/or digital data. If three time slots are used, the data rate is 192 kbps. As shown in FIG. 2 of the patent, signalling and control information in the first word of the 125 microsecond frame is transmitted with the PCM voice data and the remainder of the 125 microsecond frame. The system described in this patent illustrates the state of the art known prior to the system of the present invention, as control and/or signalling information is transmitted every 125 microseconds. In contrast, in the present invention, in order to save processing time, control information is exchanged every 625 microseconds, as will be described below in detail, and is packaged along with information on three voice and digital data buses.
FIG. 3 of the '757 patent shows how 32 of the frames shown in FIG. 2 are packaged in a superframe of 32 frames, having a total transmission time of 4 milliseconds. In the first frame, synchronizing information and the first bit of signalling information is transmitted. In the following 8 frames, 8 bits of signalling information are transmitted in the same bit position. In the remaining 23 frames, stop bits are transmitted in the same bit position. With this arrangement, the patent indicates, one signalling byte is transmitted to and from the microcomputer every 4 milliseconds. During the time that the tenth through 32nd frames are transmitted and received, the microcomputer has time to control other functions of the telephone apparatus. Accordingly, this patent relates to the packaging of PCM voice data or digital data with synchronizing and signalling information, such that the microcomputer is freed up for other activities due to the transmission format. This patent indicates that the frame format facilitates the transmission of two or more independent and simultaneous voice and/or data channels within one PCM frame without any buffering. According to the patent, this format makes possible the provision of additional features, such as additional connections to peripheral units.
U.S. Pat. No. 4,280,217 discloses a time division switching system control arrangement. As shown in FIG. 6 of the '217 patent, the sampled analog signals from the telephones are converted to 8-bit PCM representations. The 8-bit PCM representations are used as parts of the data word transmitted to a time slot interchange unit 11. Each data word is 16 bits in length and comprises an 8-bit PCM data portion, a 7-bit signalling portion and a parity bit. The signalling portion is used to convey signalling information about the channel circuit or the subscriber set to which it is connected. The data words are transmitted from a channel circuit to a multiplex/demultiplex circuit which is connected to transmit and receive time multiplexed digital information to and from a time slot interchange unit. The multiplex/demultiplex circuit transmits digital information to the time slot interchange unit on a time multiplex line in 125 microsecond frames, each comprising 64 channels of 16 bits each. The multiplex/demultiplex circuit transmits digital information to the time slot interchange unit on a time multiplex line in 125 microsecond frames, each comprising 64 channels of 16 bits each. The multiplex/demultiplex circuit receives digital information from the time slot interchange unit via another time multiplex line in a format substantially identical to the format on the first time multiplex line. See col. 6, line 47 to col. 17, line 11 of the '217 patent.
U.S. Pat. No. 3,924,077 discloses a pulse code modulation time division multiplex telephone system which allows a plurality of subscriber sets to be connected at various points along a single wire pair. Each telephone contains circuitry for sending and receiving pulse coded speech samples at the proper instant within the time multiplex frame. As shown in FIG. 5 of this patent, sixteen 96 bit frames constitute a multiframe and each frame has a 125 microsecond duration. Each telephone on a line is permanently assigned a unique address corresponding to one of the 16 frames. Thus, each telephone has its own frame number within the multiframe during which it can connect itself in an available time slot.
U.S. Pat. No. 4,476,559 discloses a transmission system comprising apparatus for the transmission of voice signals, data signals or a combination of both voice and data signals in a multiplex system of 8-bit time slots over a single bidirectional digital channel. In a combined mode, the encoded voice signals are assigned to 4-bit positions of the 8-bit time slot and the data signals are assigned to the remaining 4 bits. Up to 2-bit positions normally used for data may be used for signature bits, thereby identifying whether the multiplex stream comprises voice signals only, data signals only or a combination of both voice and data signals.
U.S. Pat. No. 4,432,089 describes a digital loop transceiver for interfacing a digital private automatic branch exchange (PABX) to a digital subscriber set via a subscriber line. As shown in FIG. 6 of this patent, a frame of data transmitted to and from the telephone comprises an 8-bit digital data word for the first channel, an 8-bit digital data word for the second channel and the signalling bits of the first and second channel. As shown in FIG. 6 of this patent, the first channel may be digital data and the second channel may comprise voice information.
The above patents show various telephone systems, some of which allow the transmission of voice data and digital data. None of these systems, however, provide a relatively inexpensive system for the transmission of both digitized voice signals and computer digital data as contemplated by the present invention.